Experimental investigation of saturated degenerate four-wave mixing for quantitative concentration measurements

摘要

Degenerate four-wave mixing (DFWM) line shapes and signal intensities are measured experimentally in well-characterized hydrogen-air flames operated over a wide range of equivalence ratios. We use both low (perturbative) and high (saturating) beam intensities in the phase-conjugate geometry. Resonances in the A~(2)∑~(+)-X~(2)II (0,0) band of OH are probed with multiaxial-mede laser radiation. The effects of saturation on the line-center signal intensity and the resonance linewidth are investigated. The DFWM signal intensities are used to measure OH number densities in a series of near-adiabatic flames at equivalence ratios ranging from 0.5 to 1.5. Use of saturating pump intensities minimizes the effects of beam absorption, providing more-accurate number density measurements. The saturated DFWM results are in excellent agreement with OH absorption measurements and equilibrium calculations tions of OH number density. The polarization dependence of the P~(1)(2) and R_(2)(1) resonances is investigated in both laser intensity regimes. There is a significant change in relative refiectivities for different polarization configurations when saturated.